Python Style Guide¶

This document is a brief set of guidelines for writing Python code at Mozilla. Individual projects may override these rules; make sure you know the standards for your project!

General Guidelines¶

Follow PEP 8.
Check your code against a linting tool. flake8 is highly recommended for this.

Import Statements¶

We expand on PEP 8‘s suggestions for import statements. These greatly improve one’s ability to ascertain what is and isn’t available in a given file.

Import one module per import statement:

import os
import sys

not:

import os, sys

Separate imports into groups with a line of whitespace: standard library; (if a web app) Django or other framework; third-party; and local imports:

import os
import sys

from django.conf import settings

import pyquery

from myapp import models, views

Alphabetize your imports; it will make your code easier to scan. See how terrible this is:

import cows
import kittens
import bears

A simple sort:

import bears
import cows
import kittens

Imports on top, from imports below:

import x
import y
import z
from bears import pandas
from xylophone import bar
from zoos import lions

That’s loads easier to read than:

from bears import pandas
import x
from xylophone import bar
import y
import z
from zoos import lions

Lastly, when importing things into your namespace from a package use an alphabetized CONSTANT, Class, var order:

from models import DATE, TIME, Dog, Kitteh, upload_pets

If possible though, it may be easier to import the entire package, especially for methods as it help answers the question, “where did you come from?”

Bad:

from foo import you


def my_code():
    you()  # wait, is this defined in this file?

Good:

import foo


def my_code():
    foo.you()  # oh you...

Whitespace matters¶

Use 4 spaces, not 2—it increases legibility considerably.
Never use tabs—history has shown that we cannot handle them.

Use single quotes unless double (or triple) quotes would be an improvement:

'this is good'

'this\'s bad'

"this's good"

"this is inconsistent, but ok"

"""this's sometimes "necessary"."""

'''nobody really does this'''

To continue a new line use a () not \.

Indenting code should be done in one of two ways: a hanging indent, or 4-space indent on the next line.

Good, using hanging indent. Note that the next line is lined up with the previous line delimiter:

log.msg('Something long log message and some vars: {0}, {1}'
        .format(variable_a, variable_b))

Good using 4 spaces:

accounts = PaymentAccounts.objects.filter(
    accounts__provider__type=2,
    something_else=True
)

# A more compact alternative.
accounts = PaymentAccounts.objects.filter(
    accounts__provider__type=2, something_else=True)

accounts = (PaymentAccounts.objects
    .filter(accounts__provider__type=2)
    .exclude(something_else=False)
)

Remember that comprehensibility is the goal here. If following one of the rules above would result in less readable code, don’t follow it!